Steering of a vehicle is achieved in such a manner that an operation of a steering member (in general, an operation of rotating a steering wheel) performed by a driver is transmitted to a steering shaft in a steering mechanism so that the operation of the steering shaft steers wheels for steering (in general, right and left front wheels).
As a vehicle steering apparatus for performing such steering, an electric power steering apparatus is practically realized in which a steering motor is attached to a steering mechanism linked mechanically to a steering member so that a torque of the steering motor driven in accordance with an operation of the steering member is transmitted to a steering shaft in the steering mechanism and thereby assists the steering performed by mechanical transmission from the steering member to the steering shaft. Further, on the other hand, a separate type steering apparatus, that is, a steering apparatus of so-called steering-by-wire type, is under development in which a steering motor is attached to a steering mechanism separated mechanically from a steering member so that a torque of the steering motor driven in accordance with an operation of the steering member is solely transmitted to a steering shaft in the steering mechanism and thereby achieves steering.
In many cases, the steering mechanism of a vehicle is provided with a steering shaft extending to the right and left of the vehicle and moving in the axial direction so that the displacement of the steering shaft is transmitted to wheels for steering (in general, right and left front wheels) linked to both ends and thereby achieves steering. When a steering motor is attached to such a steering mechanism, in the inside of a housing for supporting the steering shaft, a rotating cylinder is supported in a freely rotatable manner coaxially to the steering shaft. Then, a screw mechanism such as a ball screw mechanism is constructed between the rotating cylinder and the steering shaft, so that rotation of the steering motor is transmitted to the rotating cylinder via a gear transmission unit employing spur gears, bevel gears, and the like. Then, the rotation of the rotating cylinder is motion-converted by said screw mechanism and then transmitted to the steering shaft, so that the steering shaft is moved in the axial direction (see, for example, Japanese Patent Application Laid-Open No. 2003-252212).